For computer tomography X-ray tubes are known in which the position of the focal spot on the anode changes periodically. The changing of the position of the focal spot is realized therein, for example by a magnetic deflection unit. Such an X-ray tube requires a comparatively long deflection path, i.e. a comparatively long distance between the anode and the cathode. The shorter this distance (and the higher the maximum tube voltage), the higher the deflection power will be. For the short distances occurring between the anode and the cathode in a rotary anode X-ray tube, such deflection is hardly possible.